4 Answers
4

If you SSH into your Ubuntu box, you're getting an interactive login shell. Here's the difference:

Interactive vs. non-interactive: Any shell where you can type at a prompt is interactive. In fact, many scripts test for the variable $PS1 which holds the prompt string to find out whether they're interactive. If a shell is executing a shell script, it's non-interactive.

So, if you do ssh yourbox.example.com, you'll get an interactive shell, asuming default settings, while if you do ssh yourbox.example.com mighty_shellscript.sh, you'll end up with a non-interactive shell and your SSH session will terminate when the script terminates.

Login vs. non-login: When you log in from the console or remotely (such as SSH), or when you pass the -l option to bash, you get a login shell. Otherwise--such as when you open up a terminal window--you get a non-login shell.

To test whether a shell is a login shell, check whether its command name is -bash instead of bash:

You get an interactive login shell. But don't take it for granted, check it yourself.

This tells you that you have a login shell (from man bash):

# shopt | grep login
login_shell on

This tells you that you have an interactive shell, look for the i (from man bash):

# echo $-
himBH

The interactive login shell you get has read /etc/profile and than one of ~/.bash_profile, ~/.bash_login and ~/.profile, as explained in man bash:

When bash is invoked as an interactive login shell, or as a
non-interactive shell with the --login option, it first reads and
executes commands from the file
/etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that
order, and reads and executes com‐
mands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.

This answer is incorrect. Booting has no effect whatsoever on the value of $SHELL. Plus, this answer doesn't answer the OP's question about what type of shell fires up (interactive login in the OP's example).
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Scott SeveranceSep 26 '12 at 7:58

@ScottSeverance ok answer may be wrong but what is This "Booting has no effect whatsoever on the value of $SHELL" .... Now change your shell to tcsh after changing shell reboot system and now type $SHELL. it will give /bin/tcsh... see here [max@localhost ~]$ echo $SHELL /bin/tcsh...
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maxSep 26 '12 at 9:16

So, if you use chsh to change the default shell, of course the change will be visible after rebooting, just like if I save a file, the change will be visible after rebooting. But rebooting isn't what applies the change. Any login shell will notice the change. That means you can log out and back in, or reboot, or run gnome-terminal -x bash -l, or use one of many other methods.
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Scott SeveranceSep 26 '12 at 23:09